The invention relates to machine elements and mechanisms, but more particularly, the invention relates to an improved sprocket for use with positive drive belts of the integrally molded type with transversely oriented belt teeth having oppositely facing curvilinear driving surfaces.
A type of positive drive belt is disclosed in U.S. Pat. No. 3,756,091 to Miller. The belt has teeth with generally round cross sections where the driving surfaces of the teeth are defined by two circular intersecting arcs. The belt teeth mesh with a special sprocket with mating conjugate teeth. The sprocket distributes a driving load to the belt along the entire driving surfaces of the belt teeth to affect a stress distribution where the sprocket teeth are matched to a one-half order isochromatic fringe of the elastomeric belt teeth. This means that a resultant driving load vector applied to the belt teeth must be at a substantially large angle in relation to a perpendicular from the belt tensile member. Such a driving load vector distribution is discussed in U.S. Pat. No. 3,977,265 to Worley et al which is hereby incorporated by reference.
Another sprocket for use with a belt having oppositely facing curvilinear driving surfaces is disclosed in the Worley et al. patent. While the sprocket of the invention may be used with the Worley et al. type belt, the background of this invention more appropriately concerns problems associated with the Miller-type belts and sprockets.
The Miller-type sprocket is required to have a shape that is conjugate to curvilinear surfaces of teeth of a positive drive belt. The requirement that the sprocket teeth be conjugate with the belt teeth presents problems in some drive applications where foreign material may lodge between the sprocket and belt teeth. Foreign material wedged between the sprocket and belt teeth has several deleterious effects. The foreign material has the affect of changing the sprocket pitch which would cause a poor fit with the sprocket teeth HICH, IN TURN, MAY RESULT IN EROSION OF THE ELASTOMERIC BELT TEETH. Should the foreign material be abrasive in nature, either or both the sprocket teeth and belt teeth may be eroded to the point of causing a drive system failure. Lodged foreign material has the affect of increasing sprocket diameter which could result in over tensioning the belt, or it could cause sufficient tension in the belt to overload sprocket shaft bearings to the point of their failure. A motorcycle drive is an example of a positive drive system application where either abrasive or non-abrasive foreign material may accumulate and lodge between the sprocket and belt teeth to induce the above-discussed problems. A motorcycle belt may be required to operate in an oil present environment where oil can accumulate between the sprocket and belt teeth. The presence of oil between the teeth defeats many benefits of the Miller drive. Also, abrasive particles such as dirt, sand or the like may become lodged between the belt sprocket teeth in a motorcycle drive.
Nevertheless, the Miller-type belt has many design features which offer certain advantages in some positive drive applications. The round belt teeth are suitably large to receive high sprocket loading in high torque drives. Also, a "round" tooth has sufficient depth to exhibit anti-ratcheting qualities at higher tension ratios (e.g., greater than 5:1) such as may be encountered in a fixed two-point drive which may become loosened through belt tension decay. The purpose of this invention is to provide a sprocket and a drive system adaptable for use in foreign material environments even though it is realized that when a round tooth belt is used, even loading of the belt teeth across their curvilinear driving surfaces by a conjugate sprocket is not realized.